The assassination comes at a time of improved relations and increased cooperation between Russia and Turkey. Leaders of both countries vowed to not let the assassination cast a shadow over that friendship. It also comes during a pivotal moment in Syria, where Russia has been instrumental in President Bashar al-Assad's push to retake rebel-held parts of Syria.
In televised remarks, Russian President Vladimir Putin said the killing was a clear "provocation" aimed at undermining not just the normalization of Russia-Turkish relations but the "peace process in Syria" promoted by Russia, Turkey, Iran and other countries.
"The only response we should offer to this murder is stepping up our fight against terror, and the criminals will feel the heat," he said.
Turkish President Recep Tayyip Erdogan echoed Putin's sentiments, saying the assassination was intended to destroy the normalization of Turkey-Russian relations.
"But the Russian government and the Turkish republic have the will to not fall into that provocation."
What we know
The gunman, identified as police officer Mevlut Mert Altintas, was killed in the attack, according to various news organizations including Reuters. CNN initially reported the officer was off-duty at the time of the shooting, but Turkish government statements left his duty status Monday unclear. He was born in 1994 in the town of Soke in Turkey's Aydin province, Turkish Interior Minister Suleyman Soylu said.
The attack occurred at the Cagdas Sanat Merkezi modern arts center in the heart of Ankara, a posh neighborhood of luxury hotels and foreign embassies -- including the US and Russia -- a short walk to Turkish parliament. Karlov, envoy to Turkey since 2013, was invited to speak at a photography exhibit opening featuring the work of Turkish photographers in the Russian countryside.
The longtime diplomat had begun to speak when Altintas, wearing a dark suit tie, fired shots in rapid succession, according to multiple witness accounts.
The ambassador fell to the floor. The gunman circled his body, visibly agitated as he smashed photos hanging on the wall, said Associated Press photographer Burhan Ozbilici
, who captured the incident.
"Allahu akbar (God is greatest). Do not forget Aleppo! Do not forget Syria! Do not forget Aleppo! Do not forget Syria!" Altintas is heard shouting in video of the incident.
"Only death will remove me from here. Everyone who has taken part in this oppression will one by one pay for it," he said.
A Russian investigative team is scheduled to arrive in Turkey on Tuesday to assist local authorities, Foreign Minister Sergey Lavrov said.
"The important thing is to understand who is behind this crime," he said. "We are convinced that the main goal of those who planned this barbaric act [is] to undermine the process of normalization of relations between Russia and Turkey, largely in order to prevent an effective fight against terrorism in Syria. This goal is futile. It will not work."
In a separate incident hours later, Turkish police arrested a man who fired into the air with a shotgun outside the US Embassy in Ankara, according to Turkey's state news agency Anadolu. Video fed by Turkish video news agency IHA showed a handcuffed man being led by security officers into an unmarked police car as he shouted "I swear to God. Don't play with us," in Turkish.
Focus on Syria
The shooting occurred a day before diplomats from Turkey, Iran and Russia are scheduled to meet in Moscow to discuss the situation in the besieged Syrian city of Aleppo
. Lavrov said the meeting will go as planned.
Russia is the most powerful ally of Assad's regime. It has carried out airstrikes since September 2015 to prop up the embattled leader. As one of the five permanent members of the UN Security Council, Russia has also used its veto powers to block a political solution to end the war.
"Our task is to stabilize the legitimate government and to create conditions for a political compromise ... by military means, of course," Putin told the state-run Russia 24 TV in September
Moscow has recently tried to distance itself from the current assault in eastern Aleppo, saying earlier this month it hasn't bombed the city since October 18
Turkey's role in Syria
Turkey's involvement in Syria is complicated. On one hand, the Turks are eager to help eradicate ISIS from the war-torn country. On the other hand, Ankara has worked to drive Kurdish fighters from the Syrian side of the border, fearing that an entrenched Kurdish canton there would fuel momentum for an independent Kurdish state that could claim Turkish territory.
Meanwhile, the United States supports Kurdish groups in both Syria and Iraq as critical partners in the battle against ISIS.
But Turkey, a key US ally, considers one of those groups -- the People's Protection Units, or YPG -- to be terrorists, and says it killed as many as 200 YPG fighters in airstrikes in al Bab, northern Syria, in October.
Turkey and the YPG share a common enemy in ISIS. But Turkey says the YPG is linked to Turkey's own Kurdish insurgents -- the outlawed Kurdistan Workers' Party, or PKK, which Turkey has designated a terror group and has been fighting for years.
The United States, on the other hand, draws a distinction between the YPG and the banned PKK. It has provided significant support to the YPG as a critical fighting force against ISIS and other Islamist groups in volatile northern Syria.
This has complicated the relationship between Washington and Ankara, its NATO ally and key partner in the fight against ISIS.
Turkish-Russian relations were improving
The shooting of the ambassador is the latest in a long string of attacks in Turkey this year, although it's the first in recent memory to be directed at a foreign dignitary.
The attack comes at a time where relations between the two countries had been improving -- relations that hit an all-time low after Turkish forces shot a Russian warplane out of the sky near the Syrian border in November 2015
Russia slapped a raft of sanctions on Turkey after the deadly incident, hurting Turkish exports and damaging its tourism industry.
Erdogan and Putin have spoken several times on the phone in recent weeks
as they worked to reach a deal to evacuate civilians from eastern Aleppo.